Continuously variable transmissions (CVT), of the variable pulley or sheave type, employ sheave assemblies having at least one member that is movable to control the diameter at which a flexible transmitter, such as a belt or chain, operates. The transmission has an input sheave and an output sheave, both of which have an adjustable member. The transmission ratio between the input and output sheaves varies between an underdrive ratio and an overdrive ratio.
The CVT ratio is continuously variable between the extremes of the underdrive and overdrive ratios. During the underdrive ratios, the flexible transmitter is positioned at a small diameter on the input sheave and a large diameter on the output sheave. Thus, the input sheave has more than one revolution for each revolution of the output sheave. As the diameter of the input sheave increases, the diameter of the output sheave decreases until a one-to-one ratio exists across the sheaves. During the overdrive ratios, the diameter of the input sheave is maintained larger than the diameter of the output sheave. Thus, each revolution of the input sheave results in more than one revolution of the output sheave.
To accommodate the ratio variance, at least one member of each sheave is disposed to slide axially relative to the other member of the sheave. The movable member has a control piston that is pressurized to urge the member to the desire axial position at which the effective sheave diameter will provide the desired ratio. With the currently available electro-hydraulic controls, when a ratio change is effected, the piston of one of the sheaves is supplied with additional fluid, at a higher pressure, while the piston of the other sheave is exhausted of excess fluid. The exhausted fluid is returned to a transmission sump or reservoir. When a ratio change in the opposite direction is requested, the piston that was exhausted is resupplied with fluid while a portion of the fluid in the other piston is exhausted to the sump.